1. Field of the Invention
The present invention relates to a multi-channel output type all-optical time-division demultiplexing circuit which separates an optical pulse stream of a time-division multiplexed (TDM) signal, and simultaneously outputs every channel of the TDM signal to different ports. The present invention relates also to an all-optical TDM-WDM(Time-Division-Multiplexed-Wavelength-Division-Multiplexed) conversion circuit for assigning different wavelengths to each channel of time-division multiplexed signal pulse stream input from a transmission line, and outputting a stream of wavelength-division multiplexed signal pulses to another transmission line.
This application is based on patent application No.Hei09-194218 filed in Japan, the content of which is incorporated by reference.
2. Description of the Related Art
FIG. 18 shows a first configuration of the conventional all-optical time-division demultiplexing circuit (FIGS. 5 and 6 from a Japanese Patent Application, First Publication, H4-19718 (Patent Application No. H2-125176). Utilizing a fact that when time division multiplexed signals and control pulses are launched into an optical Kerr medium, signal pulses are affected by cross-phase modulation effect of the control pulses, resulting in changes in the center frequency, thus enabling the signal pulses to be demultiplexed into individual channels of the TDM signal.
In FIG. 18, time-division multiplexed signal pulses P.sub.1, P.sub.2, P.sub.3, P.sub.4 of an optical frequency .upsilon.s are input into a wavelength-division multiplexer 1 and are multiplexed with a control pulse Pc of an optical frequency of .upsilon.c and launched into an optical Kerr medium 3 having a positive nonlinear-index coefficient. In the Kerr medium 3, the center frequencies of the signal pulses are altered by the cross-phase modulation effect of the control pulse. The process of mutual interaction is illustrated in FIG. 19.
In a Kerr medium 3 of a positive nonlinear-index coefficient, cross-phase modulation of the control pulse induces a phase shift 4 in the signal pulses. The phase shift 4 is power-dependent (i.e., proportional to the intensity shape of the control pulse) and is derived by time-differentials of the control pulse intensity interacting with the signal pulses to produce optical frequency shift 5 in the signal pulses. If the so-called "up-chirp" region is utilized, where the optical frequency increases approximately linearly (refer to shaded region in FIG. 19, corresponding to the central region of the control pulse waveform), the signal pulses P.sub.1, P.sub.2, P.sub.3, P.sub.4 of an optical frequency .upsilon.s are changed into corresponding signal pulses having different optical frequencies .upsilon..sub.1, .upsilon..sub.2, .upsilon..sub.3, .upsilon..sub.4.
Such signal pulses P.sub.1, P.sub.2, P.sub.3, P.sub.4 having different frequencies can be separated in the optical wavelengh-division demultiplexer 2 into individual optical frequencies, and can be output to respective output ports at the same time, providing an all-optical time-division demultiplexing circuit.
FIG. 20 illustrates a second configuration of the conventional all-optical time-division demultiplexing circuit disclosed in FIGS. 1 and 3 of a Japanese Patent Application, First Publication, H7-160678. In this device, time-division multiplexed signal pulses and chirped control pulses are input into a nonlinear optical loop mirror (Sagnac interferometer) based on Kerr medium 3, and the control pulse, phase-shifted by the cross-phase modulation effect in the Kerr medium, is demultiplexed to be output to individual channels of the TDM signal.
In FIG. 20, a control light source 7 is connected to an input port 6A of an optical coupler 6, and the output ports 6C, 6D are connected in a loop by way of an optical wavelength-division multiplexer 1 and a Kerr medium 3, and an optical wavelength-division demultiplexer 2 is connected to an input port 6B.
Time-division multiplexed signal pulses P.sub.1, P.sub.2, P.sub.3, . . . , P.sub.N of an optical frequency .upsilon.s are input, through an optical amplifier 8, into the wavelength multiplexer 1. Control light source 7 produces a control pulse Pc which is linearly chirped, and whose pulse duration is sufficient to include the signal pulses P.sub.1, P.sub.2, P.sub.3, . . . , P.sub.N. The control pulse Pc is launched into input port 6A of the optical coupler 6 to be divided into two signals which are output from output ports 6C, 6D and propagates through the loop in opposite directions, as clockwise (c) component and a counter-clockwise (cc) component. In the meantime, signal pulses launched into the loop from the optical wavelength-division multiplexer 1 propagates clockwise. In the Kerr medium 3, the phase of the clockwise control pulse, propagating with the clockwise signal pulses, is affected by the cross-phase modulation effect with the signal pulses. Therefore, when the c-control pulse and cc-control pulse is multiplexed again in the optical coupler 6, the control pulse, overlapped by the signal pulses and having a phase difference of .pi., is output from the input port 6B.
Accordingly, signal pulses P.sub.1, P.sub.2, P.sub.3, . . . , P.sub.N modulate corresponding control pulses P.sub.C1, P.sub.C2, P.sub.C3, . . . , P.sub.CN. These control pulses P.sub.C1, P.sub.C2, P.sub.C3, . . . , P.sub.CN are shifted in the order of the corresponding optical frequencies .upsilon..sub.1, .upsilon..sub.2, .upsilon..sub.3, . . . , .upsilon..sub.N, thereby enabling to be separated in the wavelength demultiplexer 2 into individual optical frequencies. In other words, time-division multiplexed signals are separated into each channel, thereby enabling the all-optical time-division demultiplexing circuit to output demultiplexed signal pulses to different output ports simultaneously.
FIG. 21 illustrates a third configuration of the conventional all-optical time-division demultiplexing circuit disclosed in FIGS. 6 and 7 of a Japanese Patent Application, First Publication, H7-208258, (Priority Patent Application No. H6-191645). In this circuit, time-division multiplexed signal pulses and chirped control pulses are input into a non-linear optical medium, and the TDM signal pulses, produced as a result of four-wave-mixing, are demultiplexed.
In FIG. 21, time-division multiplexed signal pulses P.sub.1, P.sub.2, P.sub.3, . . . , P.sub.N of an optical frequency .upsilon.s are input into a wavelength-division multiplexer 1. The control light source 7 produces a light whose optical frequency changes monotonically with time, and generates a control pulse Pc of a duration sufficiently long to include signal pulses P.sub.1, P.sub.2, P.sub.3, . . . , P.sub.N. Signal pulses P.sub.1, P.sub.2, P.sub.3, . . . , P.sub.N and the control pulse Pc are multiplexed in the wavelength-division multiplexer 1 and are launched into a non-linear optical medium 9.
Here, optical frequency components of the control pulse Pc synchronized with the signal pulses P.sub.1, P.sub.2, P.sub.3, . . . , P.sub.N of an optical frequency .upsilon.s are designated by .upsilon..sub.1, .upsilon..sub.2, .upsilon..sub.3, . . . .upsilon..sub.N. In this case, four-wave-mixing effect is induced in the non-linear optical medium 9 by the control pulse Pc interacting with signal pulses of different optical frequencies, and generates frequency-converted optical pulse Fi of an optical frequency .upsilon..sub.Fi (=2.upsilon.s-.upsilon.i) or an optical pulse Fi' of an optical frequency .upsilon..sub.Fi' (=2.upsilon.i-.upsilon.s), where i=1,2,3, . . . ,N.
Accordingly, frequency-converted pulses F.sub.1, F.sub.2, F.sub.3, . . . , F.sub.N or F.sub.1 ', F.sub.2 ', F.sub.3 ', . . . , F.sub.N ' are generated to correspond to signal pulses P.sub.1, P.sub.2, P.sub.3, . . . , P.sub.N, and each frequencies can be separated with a wavelength-division demultiplexer 2. In other words, an all-optical time-division demultiplexing circuit is produced that enables time-division multiplexed signals to be separated into each signal channel and output to different output ports at the same time.
The first and third circuit configurations of the all-optical time-division demultiplexing circuit shown above are also shown in a Japanese Patent Application, First Publication, H8-307391 (Patent Application No. H7-129633) as "related arts" in FIG. 6, and as "elements of the invention" in FIGS. 1 and 2.
Problems with the conventional demultiplexing circuits are outlined below.
In the first circuit configuration shown in FIGS. 18 and 19, the only useful region of the control pulse waveform is the central region where the optical frequency increases approximately monotonically with time, therefore, it is not possible to separate those time-division multiplexed pulse signals which are processed outside of the effective chirp duration. Also, because attempts are made to produce a high degree of phase shift by using a control pulse having a wide pulse width, it means that the optical power has to be quite high, requiring several watts up to several tens of watts (refer to Electron, Lett., vol. 28, pp. 1070-1071, 1992). Furthermore, separated optical pulses are produced by shifting the signal pulse frequency so that signal pulses having the original frequency are not generated.
In the second circuit configuration shown in FIG. 20, it is necessary to construct a non-linear optical loop mirror (Sagnac interferometer), which is more complex compared with other conventional circuits in which the signal pulses and control pulses are propagated in one direction in an optical Kerr medium.
In the third circuit configuration shown in FIG. 21, because the output light is a result of four-wave-mixing of signal pulses and control pulses, there are conversion losses associated with the generation efficiency of four-wave-mixing process, resulting in a high insertion loss. Also, because there is a large shift in the bandwidths between the control/signal pulses and the four-wave-mixing pulses, it introduces another problem that a wide optical bandwidth is required to achieve time-division demultiplexing.